1. Field of the invention
This invention relates to apparatus for and methods of transferring heat.
2. Description of the prior art
Heat pumps for providing sensible heating of a fluid or other substance are known. They function by accepting heat from a source which is at a relatively low temperature and rejecting the heat at a relatively high temperature to the fluid or other substance to be heated. The source will generally be a large body of some substance at a nominally constant temperature, for example the sea, a lake, a tank or pool of water, atmospheric air, the ground, a flowing fluid, a condensing fluid or a solid. Known heat pumps of this kind comprise a closed circuit containing a refrigerant. The closed circuit comprises: a first heat exchanger (hereinafter referred to as an acceptor) for heat exchange between the source and refrigerant to heat the refrigerant; a compressor for receiving the refrigerant from the acceptor and raising its temperature by the addition of mechanical work; a second heat exchanger (hereinafter referred to as a rejector) for heat exchange between the refrigerant from the compressor and the substance to be heated; and an expansion device connected between the rejector and the acceptor to cool the refrigerant from the rejector to below the source temperature.
The above-described known heat pumps generally employ a refrigerant which is at a subcritical pressure throughout the thermodynamic cycle, that is to say at all places in the closed circuit. The refrigerant accepts heat by two-phase boiling or evaporation and rejects heat by three processes, namely gas de-superheating, two-phase condensation and liquid subcooling. Consideration of the thermodynamic efficiency of the known heat pumps shows that there are two major causes of inefficiency, namely (i) entropy gain in the rejector and (ii) non-isentropic compression of the refrigerant.